Coin validators which discriminate between coins of different denominations are well known and one example is described in our GB-A-2 169 429. This coin validator includes a coin rundown path along which coins pass edgewise through a coin sensing station at which a series of inductive tests are performed on the coins with sensor coils in order to develop sensor signals which are indicative of the size and metallic content of the coin under test. The sensor signals are digitised so as to provide coin data, which are then compared with stored data by means of a microprocessor to determine the acceptability or otherwise of the coin under test. If the coin is found to be acceptable, the microprocessor operates an accept gate so that the coin is directed to an accept path. Otherwise, the accept gate remains inoperative and the coin is directed to a reject path.
The stored data are representative of acceptable values of the coin data. The stored data in theory could be represented by a single digital value but in practice, the coin parameter data varies from coin to coin, due to differences in the coins themselves and consequently, it is usual to store the data as window data corresponding to windows or ranges of acceptable values of the coin data.
The window data needs to vary from validator to validator due to minor manufacturing differences that occur between validators manufactured to the same design. Consequently, it is not possible to program a fixed set of window data into mass produced coin validators of the same design. A conventional solution to this problem is to calibrate the validators individually by passing a series of known true coins of a particular denomination through the validator so as to derive test data from which appropriate window data can be computed and stored in the memory of the validator. Reference is directed to GB-A-1 452 740. This calibration method is however, time consuming because a group of test coins for each denomination needs to be passed through the validator in order to derive data from which the windows can be computed.
Another calibration method is described in EP-A-0 072 189. In this method, first and second tokens in the form of metal discs are passed through the validator and subject to the same inductive tests as coins to be validated. The tokens are chosen to have different characteristics to the coins to be validated. During set up of the validator, the tokens are passed sequentially through the inductive sensing station and the resultant data are then compared with standard values from which calibration factors are calculated. A series of standard acceptable values of the coin data are provided and the calibration factors are applied to the standard data to derive suitable compensated values of acceptable coin data to be stored in the memory of the individual validator being calibrated.
A calibration tool is disclosed in U.S. Pat. No. 5,495,931, which is inserted into the coin rundown path. The tool includes a coil which is energisable to induce signals to the sensor coils which emulate a coin and can be used to calibrate the validator. Reference is also directed to EP-A-0 602 474 which discloses a calibration method that uses calibration discs, and a calibration algorithm in the form of a Taylor series.
These prior methods suffer a number of disadvantages. The use of calibration discs has the disadvantage that the calibration data derived from the inductive tests is produced in response to the disc rolling through the validator, which limits the accuracy that can be obtained. Furthermore, the standard values of true coins that are compensated according to the calibration factors, are not necessarily accurate. The actively energised calibration tool may not in practice provide consistent results due to differences in inductive coupling, from validator to validator.
The present invention seeks to overcome these problems.